There is previously known motor-driven reinforcing bar binders which are provided with a binding wire feeding mechanism for feeding a binding wire being bent in a loop shape to be wound over a reinforcing bar and a binding wire twisting mechanism for twist and binding the binding wire wound around the reinforcing bar, wherein a binding wire feeding operation and a binding wire twisting operation are successively performed by a trigger operation to perform a once-cycle binding operation (see, e.g. JP-U-05-003494 and JP-A-2003-064876). These reinforcing bar binders include a single-motor type in which the binding wire feeding mechanism and the binding wire twisting mechanism are driven by a single motor and a two-motor type in which the binding wire feeding mechanism and the binding wire twisting mechanism are driven by different motors, respectively. However, in either type, the operation of twisting the binding wire is subjected to considerable load and the motor in charge of the twisting operation generates a large quantity of heat.
As a means for dissipating the heat generated by an electric tool, there is a previously known method in which the interior of a housing inclusive of the motor is cooled during motor rotation by a wind supplied by a fan attached to a motor shaft (see, e.g. JP-U-07-007805). However, in the reinforcing bar binder, in which the operation pattern of the motor is different from that of the electric tool such as an electric drill or an electric driver, a sufficient motor cooling effect cannot be obtained by the fan which rotates simultaneously with the motor. Therefore, the reinforcing bar binders have not been specially provided with cooling means such as fans.
The operation pattern of the reinforcing bar binder is a short-time high speed operation in which the one cycle consisting of the operation of the binding wire feeding mechanism and that of the binding wire twisting mechanism is not longer than 1 sec. More specifically, the operation of the twisting mechanism includes an operation of closing a tip of the twisting mechanism so as to grasp the binding wire, a twisting operation of rotating the twisting mechanism and an operation of reversing the motor to release the binding wire so that the twisting mechanism is restored to an initial position. The motor changes between a normal rotation and a reverse rotation in a short period. Therefore, even with the fan attached to the motor shaft, the rotation time within the one cycle is short. In addition, the rotation of the fan is reversed soon and the number of rotations is also few. Thus, a satisfactory cooling effect cannot be expected. Accordingly, in the conventional reinforcing bar binder, in place of a cooling device, a protecting circuit using a temperature detecting element is provided so that when the temperature within a housing reaches a predetermined upper limited temperature, a power source circuit is interrupted to prevent the motors, circuit elements or the housing from being burned.
As described above, since the conventional reinforcing bar binder is not specially provided with the cooling mechanism, if the binding operation is performed continuously for a long period, the interior temperature gradually rises. As a result, the protecting circuit is activated to interrupt the power source. In this case, the reinforcing bar binder cannot be employed until the interior temperature falls to a safety reference value. In a large-scale construction scene such as bridge or road construction, although the reinforcing bar binder is continuously driven all day long, it cannot be employed owing to temperature rise so that working must be interrupted. This may leads to delay in a work schedule.
In order to obviate such an inconvenience, any cooling means is required. For example, if the reinforcing bar binder is simply provided with an electric fan connected to a power switch, the electric fan which always rotates consumes a large quantity of electric power. This leads to an inconvenience of shortening the duration of a battery of the reinforcing bar binder. Further, in a very low temperature environment in which the atmospheric temperature is not higher than 0° C., the viscosity of lubricant applied to a rotating mechanism portion of the reinforcing bar binder lowers to increase the operation resistance and reduce the battery capacity. So, unless an warm-up is carried out to some extent, the primary performance cannot be shown. In the electric fan which always rotates, air is sent in a low temperature environment also. This leads to a problem that the interior of the reinforcing bar binder is cooled to the temperature not higher than a suitable operation temperature to deteriorate the performance.